The present invention relates to a method and an apparatus for determining representative tension forces in an installed, threaded fastener.
When using a threaded fastener to create a bolted joint to connect multiple components, the clamp load created by the bolted joint is critical. The clamp force created by the bolted joint is directly proportional to the tension in the bolt. One way of indirectly measuring or auditing the clamp force or tension is by measuring the torque value it takes to break the threaded fastener loose.
Typically, a torque reading is obtained with a bending beam or dial indicator-type torque wrench. While this technique is still used today for non-critical applications, the readings obtained are known to have little or no correlation with the actual tension developed by the installation torque. Other than the fact that, in general, higher values of breakaway torque indicate that higher installation torque may have been applied, the breakaway torque measurement cannot be relied upon to verify fastener tension.
However, torque readings are not always useful. This is because there are anywhere from 75 to over 200 factors that effect the tension in a bolt when a tightening torque is applied. The main variable that effects the amount of tension in a bolt is friction. Friction can be present underneath the bolt itself, so called under head friction or friction at the threads. A typical construction of a bolted fastener is illustrated in FIG. 1.
Problems associated with measuring torque in bolted assemblies is increased by power tools which are used to deliver torque to turn fasteners in a controlled fashion. Such power tools include impact wrenches and other pulse tools. Unfortunately, the limitations of these tools relate to their energy transfer characteristics which are not generally well understood. In this respect, the tightening results in terms of the torque/tension for achieved clamp force are very much dependent on the joint friction. Pulse and impact tools are particularly sensitive to joint rate and friction variations. Since friction coefficients are a function of velocity as well as surface pressure, tightening results with pulse and high RPM tools can lead to a wide range of actual clamp forces for a given torque value. Pulse and impact tools move faster at high speeds with a great deal of stick-slip, chatter and unique frictional characteristics that are not seen with steady continuous tightening. These factors can lead to a deceptively high torque reading but with minimal clamp force created.
In 1968, the first rotary socket wrench torque transducers and the first battery powered peak meters were introduced. This allowed a comparison of dynamically applied torque to hand torque audits. Thus, the techniques for measurement and interpretation of torque signatures became highly refined and capabilities were developed to interpret both tool torque control and other factors.
Since the desired result in a bolted joint is proper fastener tightness or tension, the ideal audit process would directly determine the tension of the fastener. This can be done by attaching strain gauge to the bolt or using a force washer. However, since it is not practical to put strain gauges on production assemblies or use force washers under the head of each bolt, other techniques had to be developed. Ultrasonic techniques for measuring bolt stretch have proven useful in some applications, but there are practical limitations for this method.
If the friction coefficients and other variables associated with a given bolted joint are uniform and repeatable, it is possible to demonstrate that fastener assemblies tightened to a specified torque will achieve relatively uniform clamp loads to provide reliable performance. However, friction variables are often non-uniform and cannot be repeated. Further, these sophisticated torque measurements do not provide an accurate measurement of clamp force after installation.
While threaded fastener design procedures in software are available to assist engineers in designing bolted joints, engineers have heretofore had no practical, accurate way of determining whether the installed fastener meets the preload/tension design requirements. Therefore, a need has arisen to develop a practical, cost effective technique whereby the tension in a threaded fastener can be more accurately determined, and a method of accurately determining the residual tension on a bolted connection after a period of use.
The present invention provides a method and apparatus for auditing a tension value in an installed threaded fastener. The method includes applying a torque to a threaded fastener until a torque is reached to rotate the fastener. The torque applied to the fastener is measured as well as the angle through which the fastener rotates. These values are plotted against one another such that the torque values are plotted on a torque axis and the angle values are plotted on an angle axis. A tangent is extended from the plot at a point where the fastener began rotating towards the angle axis. The point at which the tangent line crosses the angle axis is defined to be zero degrees and the angle axis is then scaled based upon the known angle of rotation. The angle from the zero degree point to the angle corresponding to the torque necessary to rotate the fastener is directly proportional to the tension or preload in the fastener.
The invention also provides for an apparatus to audit tension in a threaded fastener. The apparatus includes a wrench operative to engage the fastener, a torque transducer which generates a signal characteristic of the torque applied to the fastener and an angle transducer which generates a signal characteristic of the angle through which the fastener rotates in response to the applied torque. A processor receives the signals from the torque and angle transducers and is configured to locate the elastic origin for the threaded fastener and determine a M-alpha angle. The M-alpha angle is directly proportional to the tension in the fastener and the processor can compare it against the predetermined M-alpha angle to determine whether the audited fastener is developing adequate clamp force.